PTOLEMY II - CiteSeerX

MoML
instantiated in a model. Components can extend other components through an object-oriented
inheritance mechanism.
Semantics independence. MoML defines no semantics for an interconnection of components. It
represents only the hierarchical containment relationships between entities with properties, their
ports, and the connections between their ports. In Ptolemy II, the meaning of a connection (the
semantics of the model) is defined by the director for the model, which is a property of the toplevel
entity. The director defines the semantics of the interconnection. MoML knows nothing
about directors except that they are instances of classes that can be loaded by the class loader and
assigned as properties.
7.2.1 Clustered Graphs
A model is given as a clustered graph, which is an abstract syntax for representing netlists, state
transition diagrams, block diagrams, etc. An abstract syntax is a conceptual data organization. A particular
clustered graph configuration is called a topology. A topology is a collection of entities and
relations. Furthermore, entities have ports and relations connect the ports. We consistently use the term
connection to denote the association between connected ports (or their entities), and the term link to
denote the association between ports and relations. Thus, a connection consists of a relation and two or
more links.
The concept of an abstract syntax can be contrasted with a concrete syntax, which is a persistent,
readable representation of the data. For example, EDIF is a concrete syntax for representing netlists.
MoML is a concrete syntax for the clustered graph abstract syntax. Furthermore, we use the visual
notation shown in figure 7.3, where entities are depicted as rounded boxes, relations as diamonds, and
entities as filled circles.
The use of ports and hierarchy distinguishes our topologies from mathematical graphs. In a mathematical
graph, an entity would be a vertex, and an arc would be a connection between entities. A vertex
could be represented in our schema using entities that always contain exactly one port. In a directed
graph, the connections are divided into two subsets, one consisting of incoming arcs, and the other of
outgoing arcs. The vertices in such a graph could be represented by entities that contain two ports, one
for incoming arcs and one for outgoing arcs. Thus, in mathematical graphs, entities always have one or
two ports, depending on whether the graph is directed. Our schema generalizes this by permitting an
entity to have any number of ports, thus dividing its connections into an arbitrary number of subsets.
Entity
Port
FIGURE 7.3. Visual notation and terminology.
Connection
Link Link
Relation
Connection Connection
Link
Entity
Entity
194 Ptolemy II
Port
Port